Air/fuel ratio indicator



1969 c. WHITMORE ETAL 3,478,579

AIR/FUEL RATIO INDICATOR Filed Aug. 8, 1968 2 Sheets-Sheet 1 Pwm P5: 3 0m x how INVENTORS CHATLAND WHITMORE geoaee F. REDWA Y ATTORNEYS FIG. 3

Nov. 18, 1969 c. WHITMORE ET AL 3,478,579

AIR/FUEL RATIO INDICATOR Filed Aug. 8,- 1968 2 sheets sheet 2 CHATLAND WHITMORE GEORGE F. REDWAY ATTORNEYS United States Patent US. Cl. 73-116 Claims ABSTRACT OF THE DISCLOSURE The composition of exhaust gases from aircraft engines, etc. is sensed by a thermal conductivity cell supplied with constantcurrent, and the output of the cell is amplified and indicated. A regulated power supply uses a transformer with a primary winding connected in an oscillator circuit energized with DC. A rectifier supplied from a secondary winding controls the oscillator to maintain the rectifier voltage constant. Advantageously a transistor multivibrator whose bias is controlled by a separate rectifier is employed. Additional separate rectifiers connected to the transformer supply current to the sensing cell and to the indicator amplifier. A simple two-transistor constant current generator with additional control voltage regulation is described. Exhaust gas temperature may also be sensed and indicated on a meter adjacent the air/fuel meter.

Background of the invention Proper management of the air and fuel consumed by engines is important for efficiency, power, etc. In aircraft engines of the piston type the problem is particularly serious. Air pressure varies with altitude, and the rate at which air is supplied to an engine varies accordingly unless superchargers are used. Consequently it is common practice to change the rate at which fuel is supplied to the engine as the altitude changes, to preserve a proper air/fuel (A/F) ratio for existing conditions. High fuel consumption, low speed and lack of power, even at full throttle, may result at high altitude if the air/ fuel mixture is too rich. On the other hand, high cylinder head temperature, burned valves and rings, and erratic engine operation may result if the mixture is too lean.

Arbitrary settings for various altitudes based on experience, manufacturers instructions, etc. are frequently used. If followed carefully, engine damage may normally be avoided. However, it is unlikely that maximum economy and fuel range will be obtained.

For a more exact setting, exhaust gas temperature (EGT) is frequently used as a guide. One procedure is to start with the air/ fuel ratio at full rich, lean the mixture until a maximum temperature is reached, and then enrich the mixture until the temperature drops a given amount, say 100 F. This may be satisfactory at cruise power settings or greater, and at a constant altitude. However, to make this adjustment it is important to start with the mixture on the rich side in order to secure good economy without danger of damaging the engine. If the altitude changes, it may be necessary to repeat the procedure since the A/F ratio may become too lean even though the EGT reading is the same as before, inasmuch as the pilot may not notice a rise in EGT and a subsequent drop while changing altitude. Thus this procedure is not entirely satisfactory, particularly since an improper A/F setting may quickly damage the engine.

A direct indication of the A/ F ratio is far superior for proper setting of the leaning control for the existing altitude and the power required. For a given engine, the A/F ratio yielding a desired power is substantially constant regardless of altitude. Such a direct indication can be 3,478,579 Patented Nov. 18, 1969 based on sensing the composition of the exhaust gases.

One known sensor is a so-called thermal conductivity cell (TCC) including a bridge circuit having coils of platinum wire in each leg, two of the coils being exposed to the exhaust gases and the other two sealed in air saturated with moisture. The bridge is placed in a chamber supplied with the exhaust gases through conduits connected to the exhaust manifold. If the bridge is supplied with constant DC to one diagonal, the signal across the other diagonal will vary with the proportions of hydrogen (H) and carbon dioxide (CO in the exhaust. The signal may be amplified and indicated on a meter, and yields indications directly related to the A/F ratio. For example the meter may be calibrated to read 13 for a zero TCC bridge output with air in the bridge chamber, 9 for a negative signal with H in the chamber, and 15 for a positive signal with CO in the chamber, in which case the readings give the ratio of pounds of air per pound of fuel. The meter could be calibrated, if desired, to indicate percent complete combustion or in arbitrary units.

A/F indicators have been built using the thermal conductivity cell. However, they have been too heavy and expensive for wide-spread small plane use where they are likely to be most needed.

Summary of the invention The present invention provides an A/F indicator which is of relatively light weight and low cost, yet provides adequate accuracy of indication for the purpose. Although usable separately, it may be combined with an EGT meter.

Exhaust gas sensing means such as the above-described thermal conductivity cell is supplied with constant current from a constant current generator which in turn is supplied from a regulated power supply. The output of the sensing means is amplified and supplied to an indicating meter.

The regulated power supply includes a transformer having primary and secondary windings. The primary Winding forms part of an oscillator energized from a DC source, and the oscillator is advantageously a transistor multivibrator. Rectifier means supplied with AC from the transformer yields a DC output voltage which is fed back to the oscillator to control the oscillator output so as to maintain the DC voltage substantially constant. In the specific embodiment the feedback controls the bias on the multivibrator transistors, and a Zener diode in the feedback circuit establishes a constant difference between the DC rectified voltage and the bias control voltage. The rectifier means also supplies DC through the constant current generator to the sensing means, and supplies power to the indicator amplifier.

It is particularly contemplated to employ a separate rectifier circuit for controlling the oscillator output. This has the important advantage that the rectifier load circuit is fixed and unaffected by changes in load on the rectifying means which supplies power to the sensing means and amplifier, and can be designed specifically for control purposes. Yet any changes in load on the oscillator are reflected in the voltage output of the control rectifier, and prompt regulation restores the voltage to its desired value. It is also advantageous to employ separate rectifier circuits for the sensing means and the amplifier. It is found that in this manner close regulation can be obtained in an economical manner, despite changes in load on the separate rectifiers.

A further feature of the invention is a simple but satisfactory constant current generator employing a pair of transistors and supplied with current from one of the rectifier circuits of the regulated power supply. Current to the sensing means is supplied through the output circuit of one transistor and a resistor in series therewith. The

other transistor has its output circuit connected to the input circuit of the first transistor so as to control the output of the latter. The input circuit of the control transistor includes the series resistor and is responsive to any change in current therethrough to control the first transistor and restore the current to its desired constant value. An adjustable voltage, obtained from a voltage divider connected to the regulated power supply and further regulated by a Zener diode, is supplied to the input of the control transistor to permit setting the value of the constant current.

Although the specific embodiment is particularly designed for aircraft use, it may also be used in other environments where increased economy or more satisfactory operating conditions justify the use of an air/fuel indicator. One such application would be in large trucks to increase mileage and yield more satisfactory combustion.

Brief description of the drawings FIG. 1 shows the overall arrangement of the invention, providing both A/F and EGT indications;

FIG. 2 is a circuit diagram of the EGT indicator;

FIG. 3 is a circuit diagram of the regulated power supply used in the A/F indicator; and

FIG. 4 is a circuit diagram of the remainder of the A/ F indicator.

Description of the preferred embodiment Referring to FIG. 1, an arrangement is shown for monitoring two engines. Respective engine exhaust manifolds are shown at 10, Respective thermistor probes 11, 11 are inserted in the manifolds and are responsive to the exhaust gas temperature therein. Respective manifolds have two tubes 12, 13 and 12', 13' sealed in the walls thereof for bypassing a portion of the exhaust gases through chambers 14, 14. Each chamber contains a thermal conductivity cell (T CC) with electrical connections through cables 15, 15' to respective A/F indicator circuits 1'6, 16. A common regulated power supply 17 energizes the A/F circuits. Advantageously the power supply 17 has separate rectifier circuits which supply current for respective TCCs through lines 18, 18' to the A/F circuits, and the latter contain constant current generators connected to the TCCs through cables 15, 15'. Energizing voltages for amplifiers in the A/F circuits are supplied from a common rectifier in the power supply through line 19. The amplified TCC signals are supplied to respective meters 21, 21' for indication.

The thermistor probes 11, 11' are connected to respective EGT circuits 22, 22, and the outputs supplied to respective meters 23, 23'. The A/F and EGT meters are grouped as shown, for convenient observation.

EGT meters 23, 23' are shown with calibrations 12 and 18 to indicate a temperature range from 1200 to 1800 F. Suitable adjustments are described in connection with FIG. 2. A/F meters 21, 21' are shown with calibrations 9, 11, 13 and 15 to indicate pounds of air per pound of fuel, as described above. Suitable adjustments are described in connection with FIG. 4. Other ranges may of course be provided if desired.

Referring to FIG. 2, a DC voltage designated +V, from the airplane power supply, a battery or other suitable source, is applied through resistor 25 to a Zener diode 26 which maintains a constant voltage on line 27. The voltage on line 27 is applied across one diagonal of a bridge containing resistors 28, 29, 30, zero set potentiometer 31, and the thermistor probe 11. Meter 23, resistor 32, and potentiometer 33 (connected as a variable resistor) are connected in series across the other diagonal. A switch 34 and resistor 35 enable convenient calibration of the meter.

Resistor 35 may be chosen equal to the resistance of probe 11 at the lowest temperature desired to be indicated, and potentiometer 31 adjusted to give the proper meter reading. Potentiometer 33 serves as a gain set to provide the proper meter range in accordance with the scale chosen and the probe characteristic.

Referring to FIG. 3, a multi-coil transformer is generally indicated at 41. Preferably this is a toroid transformer. Two coils 42, 42' are connected together to form a center-tapped coil, and power is supplied to the center tap through blocking diode 43 from a suitable DC source designated +V. Coils 42, 42 are connected in the emitter-collector circuits of transistors 44, 44' which have their input and output circuits cross-coupled in conventional manner to form an oscillator of the multivibrator type.

To this end, the collectors are connected to respectively opposite bases through resistors 45, 46 and 45, 46', and resistors 45, 45' are shunted by respective capacitors 47, 47. The transistors here shown are of the NPN type, and diodes 48, 48' insure that the bases will not go negative to the emitters. The biasing circuits include diodes 49, 49 and diode 50, and the bias is controlled in a manner to be described below. Diode 50 is of the type having a predetermined breakdown voltage in the reverse current direction, and a Zener diode is here employed.

The circuit constants may be selected to provide a desired operating frequency, which in this specific embodiment is nominally 20 kh. Since coils 42, 42 are in the output circuits of the transistors, they serve as primary windings for transformer 41. Capacitor 40 serves as a filter capacitor to prevent an excessive AC voltage from feeding back into the +V line.

A separate secondary coil 51 on the transformer supplies the AC produced by the multivibrator to a diode bridge rectifier 52 having a load resistor 53 shunted by filter capacitor 54. This yields a DC output in line 55 of the negative polarity indicated. The DC output is fed back to the biasing circuit of the multivibrator to control the AC output thereof so that the rectifier voltage output at line 55 remains substantially constant.

Describing the control in more detail, the multivibrator initially starts oscillating with the bases of transistors 44, 44' positively biased by +V through resistors 45, 46 and 45, 46'. As capacitors 47, 47' become charged the bias on the bases decreases. As the oscillation builds up, the rectified DC voltage at line 55 increases in the negative direction until the voltage across Zener diode 50 reaches the breakdown voltage thereof, say 10 volts. Thereafter the potential of line 56 will remain approximately 10 volts above that of line 55, and the resultant voltage is effective through diodes 49, 49' to modify the bias on the bases of transistors 44, 44 to change the frequency and current until a stable operating condition is reached. It should be noted that the negative feedback provided from the rectifier circuit 51-54 to the multivibrator provides overall voltage regulation for all the secondary windings on the transformer. Thus any change in load conditions in any of the circuits connected to the secondary windings which alter the output of the multivibrator will immediately be reflected as a change of voltage at line 55. Consequently the multivibrator will be controlled promptly to restore the desired voltage at line 55.

By employing a separate rectifier circuit for the control, it can be designed in view of control considerations only, and its load remains constant regardless of changes in load on the rectifiers supplying power to the A/F circuits. This enhances effective regulation.

Separate bridge rectifiers 57, 58 supplied from the transformer by secondary windings 59, 60 yield DC outputs A-B, A'B' for respective thermal conductivity cells for the two engines. Filter capacitors 61, 62 shunt respective outputs. The separate supplies permit independent adjustment of the voltage regulators for the two cells without undesirable interaction.

Power for the amplifiers in the two A/F circuits 16, 16 is supplied by windings 63, 63', connected as a centertapped winding, and supplying AC to a bridge rectifier circuit 64. Balanced DC outputs E and C of opposite polarity are provided. Each output is filtered by an R-C filter as shown, and firmly regulated by Zener diodes 65, 65'.

Referring to FIG. 4, one of the A/F indicator circuits 16, 16' is shown, with the associated thermal conductivity cell and meter 21. Rectified power from A-B of FIG., 3 is supplied across a series chain of resistors 71-73 and potentiometer 74 which form a voltage divider. A Zener diode 75 is connected across 71, 72 and 74 to firmly regulate the voltage thereacross. Current through resistor 73biases diode 75 into regulation.

The TCC cell generally designated 78 is supplied with constant current through lines 79 and 77 under the control of aconstant current generator including transistors 81, 82. The cell 78 has platinum wire coils 83, 83' exposed to the exhaust gases, and enclosed cells 84, 84', arranged in a bridge configuration, as previously described. A variable resistor 85, across the diagonal to which output lines 86, 86' are connected, permits adjusting the bridge sensitivity. Resistor 87 across coil 83, and resistor 88 and variable resistor 89 across coil 84, per mits balancing the bridge.

The constant current generator supplies current to the TCC through resistor 91 and the output circuit of transistor 82. Transistor 81 serves as a control transistor and has an input circuit including resistor 91, and an output circuit connected to the input circuit of transistor '82. Thus control transistor 81 is responsive to changes in current flow through resistor91 and controls transistor 82 to maintain constant the current through line 79 to the TCC.

Specifically, resistor 91 is in series with the output collector-emitter circuit of transistor 82, here of the NPN type. The lower end of the resistor is connected to the emitter of control transistor 81 of opposite type, here PNP. The base of 81 is connected to the slider of potentiometer 74. The voltage across the portion of the voltage divider including potentiometer 74 is accurately maintained by Zener diode 75 so that, once the potentiometer is set, the voltage from the base of transistor 81 to line 76 will remain constant. Resistor 92 is in series with the emitter-collector circuit of control transistor 81, and is connected across the base-emitter circuit of transistor 82.

The emitter-base voltage of transistor 81 is the difference between the voltage from the slider of potentiometer 74 to line 76, and the IR drop in resistor 91. This determines the current flowing through resistor 92 and hence the base-emitter voltage of transistor 82. The latter controls the current through the collector-emitter circuit of 82 to line 79. Initial adjustment of potentiometer 74 determines the initial flow of current in line 79. Thereafter a reduction in current in line 79 is accompanied by a reduced IR drop across resistor 91, a greater input voltage to transistor 81, a larger current in resistor 92 with consequent greater input voltage to transistor 82, and a reduction in the collector-emitter impedance of 82 to restore the initial current value. The reverse takes place upon increase in current in line 79. Thus the current in line 79 is maintained constant at a value selected by potentiometer 74.

With switch 93 in its upper position, meter 21 is connected across resistor 91 through resistor 94, thereby facilitating setting the bridge current at the desired value.

The bridge output in line 86 is supplied through switch 95 to an amplifier here shown as an integrated circuit operational amplifier 97 of a commercially available meter is returned through line 103 to input 2, and to ground via the resistors 98-100. This forms a feedback circuit, and the gain of the amplifier can be adjusted by variable resistor 100.

Power is supplied to the amplifier at C and E, from the rectifier 64 in FIG. 3. Resistors 104, 104' and capacitors 105, 105' form low-pass decoupling filters. A potentiometer 106 is connected across the power lines, and the slider connected through resistor 107 to input 2 of the amplifier. By moving switch 95 to ground the amplifier input 1, and adjusting potentiometer 106, the meter zero may be set. Capacitors 108 and 109 restrict the high frequency response of the amplifier to promote stability.

The specific connections to amplifier 97 are in accordance with manufacturers specifications and need not be described further. It will be understood that, although an integrated circuit amplifier is desirable for compactness and light weight, conventional amplifier circuits using discrete components may be employed if desired.

The A/ F indicator may be calibrated by adjusting variable resistor 89 to balance the TCC bridge 78 with air in the TCC chamber 14. Potentiometer 74 is adjusted to give a desired bridge current as indicated on the meter 21 with switch 93 in its upper position, and variable resistor adjusted to give specified outputs in line 86 with air, H or CO in the TCC chamber 14. For example, the adjustments may be made to give a zero bridge output for air, 6.0 mv. for H, and +1.6 mv. for C0 The zero setting of meter 21 may be adjusted by moving switch to ground amplifier input 1, and adjusting potentiometer 106. The corresponding meter calibration may be 13 as previously described. Then the amplifier gain may be set by variable resistor to yield meter readings of 9 for 6.0 mv. and 15 for +1.6 mv. inputs.

After proper initial adjustment, in normal operation it will usually be sufficient to check the TCC bridge current with switch 93 in its upper position, and the meter zero with switch 93 in the position shown and switch 95 moved to ground, with suitable readjustments of potentiometers 74 and 106 as required.

We claim:

1. An air/fuel ratio indicator for engines including sensing means supplied with a constant DC current and yielding a DC output varying with changes in exhaust gas composition of a said engine, and indicating means responsive to said output, wherein the improvement comprises (a) an oscillator adapted to be energized from a DC source and including control means for controlling the output thereof,

(b) a transformer supplied with the output of said oscillator,

(c) rectifier means supplied from said transformer and yielding at least one DC output voltage,

(d) a feedback circuit from said rectifier means to said control means for controlling the oscillator output to maintain said DC output voltage substantially constant,

(e) a constant current generator supplied from said rectifier means and connected to supply current to said sensing means,

(f) a DC'amplifier connected to be energized from said rectifier means,

(g) means for supplying the output of said sensing means to said amplifier,

(h) and means for supplying the output of said amplifier to said indicating means.

2. An air/fuel ratio indicator according to claim 1 in which said rectifier means includes a plurality of separate rectifier circuits supplied from said transformer, said feedback circuit for controlling the oscillator output being from one of said separate rectifier circuits, and said constant current generator being supplied from another of said separate rectifier circuits.

3. An air/fuel indicator according to claim 2 in which said DC amplifier is connected to be energized from another of said separate rectifier circuits.

4. An air/fuel ratio indicator according to claim 2 in which (a) said oscillator is a multivibrator including a pair of transistors having cross-coupled input and output circuits and said control means includes biasing means for changing the bias in said input circuits,

(b) and said feedback circuit includes a diode having a predetermined breakdown voltage in the reverse current direction thereof,

() said diode being connected in series between said one separate rectifier circuit and said biasing means for controlling the multivibrator output upon the rectifier output voltage reaching a value at which the voltage across said diode reaches the breakdown voltage thereof.

5. An air/fuel ratio indicator according to claim 1 in which said constant current generator includes (a) a first transistor and a resistor,

(b) the output circuit of said first transistor and said resistor being connected in series with said sensing means,

(c) a control transistor having an output circuit connected to the input circuit of said first transistor for controlling current flow therethrough,

(d) and an input circuit for said control transistor including said resistor and responsive to change of current flow therethrough for controlling said first transistor to maintain constant the current to said sensing means.

6. An air/fuel ratio indicator according to claim 1 in which said rectifier means includes a pair of lines for supplying current through said constant current generator to said sensing means, and said constant current generator includes (a) a first resistor and a first transistor,

(b) said first resistor, the collector-emitter circuit of said first transistor and said sensing means being connected in series between said lines in the order named,

(c) a second resistor and a control transistor of opposite type to said first transistor,

(d) the emitter and collector of said control transistor being connected respectively to the collector and base of said first transistor and said second resistor being connected between the collector of the control transistor and the emitter of said first transistor,

(e) a voltage divider connected between said lines,

(f) and a connection from a point on said voltage divider to the base of said control transistor,

(g) whereby said control transistor is responsive to the voltage across said first resistor to control said first transistor to maintain constant the current through said sensing means.

7. An air/fuel ratio indicator according to claim 6 including a diode having a predetermined breakdown voltage in the reverse current direction thereof, said diode being connected across at least a portion of said voltage divider including said point of connection to the base of said control transistor to maintain constant the voltage thereacross.

8. An air/fuel ratio indicator according to claim 7 in which said point of connection is adjustable to thereby adjust the current through said sensing means.

An air/fuel ratio indicator according to claim 8 in which (a) said oscillator is a multivibrator including a pair of transistors having cross-coupled input and output circuits and said control means includes biasing means for changing the bias in said input circuits,

(b) said rectifier means includes a plurality of separate rectifier circuits supplied from said transformer,

(c) said feedback circuit includes a diode having a predetermined breakdown voltage in the reverse current direction thereof,

(d) said diode being connected in series between the output of one of said separate rectifier circuits and said biasing means for controlling the multivibrator output upon the rectifier output voltage reaching a value at which the voltage across said diode reaches the breakdown voltage thereof,

(e) and said pair of lines for supplying current through said constant current generator to the sensing means is from another of said separate rectifier circuits.

10. An air/fuel ratio indicator according to claim 9 in which said DC amplifier is connected to be energized from another of said separate rectifier circuits.

References Cited UNITED STATES PATENTS 2,591,759 4/ 1952 Zaikowsky 7327 2,782,102 2/1957 Howe 7327 X 3,370,457 2/1968 Lemm 7327 RICHARD C. QUEISSER, Primary Examiner J. W. MYRACLE, Assistant Examiner U.S. Cl. X.R. 23-254; 73--27 

